Production Technique of Synthetic Jet Actuators

ABSTRACT

A clawless synthetic jet actuator includes a cavity layer having an internal cavity for reception of a fluid volume and an orifice providing a fluid communication between the cavity and an external atmosphere; and an oscillatory membrane having a piezoelectric material adapted to deflect the oscillatory membrane in response to an electrical signal. The cavity has an opening in at least one planar surface of the cavity layer, and the cavity layer and the oscillatory membrane are joined by a high strength, low shear modulus adhesive material with the oscillatory membrane positioned adjacent to the planar surface having the cavity opening and adapted as an enclosing surface to said cavity opening. The oscillatory membrane is adapted to compress and expand a volume within the cavity, based on a deflection generated by the piezoelectric material, for generating a fluid flow between the cavity and the external atmosphere through the orifice.

FIELD OF THE INVENTION

The present invention relates to synthetic jet actuators and methods ofmaking and using the same. In particular, the present invention isdirected to improvements in synthetic jet actuators suitable for use inzero net mass flow applications.

BACKGROUND OF THE INVENTION

Piezoelectric-driven membranes have been used to generate synthetic jetsin various active flow control applications such as fatigue reduction onwind turbine blades, electronic cooling, and drag reduction for aerialand ground vehicles. Studies have shown that with these actuators,Active Flow Control (AFC) systems can dramatically change the airflowregime around a bluff body resulting in improved aerodynamicperformance.

As one example, when applied to a ground vehicle such as a tractortrailer truck, synthetic-jet-based AFC may create a virtual surfacemodification, effectively changing the airflow around the vehicle, andthus the aerodynamic drag on the vehicle, without changing the vehicleshape. Although AFC systems are known in academic research, previousattempts to apply the technology to ground vehicles has been limited byhigh sound levels, low jet speed, and low jet momentum, as well as highpower consumption by the actuators.

Conventional AFC systems have included the use of clamped actuators inzero net mass flow (ZNMF) designs. A ZNMF design is one that generates ajet flow through manipulation of a surrounding environment fluid,without requiring an additional fluid supply, such that the jet flow isgenerated without any change in the net mass of the surroundingenvironment fluid. To date, however, conventional clamped actuators havebeen found to have a number of undesirable limitations, includingperformance limitations, scaling factor limitations, and integrationfactor limitations.

Nuventix has developed clamped-edge actuators that are optimized foracoustical performance for use in cooling LED lights, however theseactuators reach very low jet speeds that are insufficient forapplications such as drag reduction in vehicles which instead requirehigh jet speeds. While laboratory testing at Rensselaer PolytechnicInstitute suggests clamped-edge actuators may be capable of achievingjet speeds in excess of 200 m/s, such actuators are considered notcommercially viable as they require the actuators to be constructed withan excessive size and mass, through complicated assembly methods withpoor repeatability and seal-ability, and with the assembled actuatorsbeing incapable of fine resonance frequency tuning (e.g., to avoid highdecibels) and requiring a high power consumption.

Other attempts at designing actuators for reducing noise levels includemodifications to the actuators that prohibit their use in manycommercial applications that require high jet velocities, integrationinto constrained spaces such as a surface of a vehicle, or the use ofmultiple actuators (an actuator array) operated simultaneously toachieve a desired outcome. One example of an actuator design intendedfor reducing noise is provided in U.S. Pat. No. 8,564,217, in whichnoise reduction is achieved for a single actuator for very low jetvelocities (<30 m/s) through use of a fixed phase angle that isapplicable in the case of an individual actuator, but which isineffective for use in an array of actuators. Another example isprovided in U.S. Pat. No. 8,308,078, in which a synthetic jet actuatoris provided with two orifices that are made to face in differentdirections in order to reduce an overall noise level generated by thejets generated therefrom, though at the expense of limited directionaljet velocity and rendering these actuators unsuitable for use in anyapplication in which directional control of the jets is required toinclude ejecting the jets in a common direction. A further example isprovided in European patent no. 2 873 609, in which the actuator isprovided with a surrounding muffler that substantially increased thevolume of the actuator and effectively prohibits the actuator from usein applications that require integration of the actuator into a confinedspace (e.g., surface integration applications).

Despite the advances in the art to date, there remains a need forimprovements to synthetic jet actuators for yet further advancing thestate of the art, and improving the ZNMF designs generally.

SUMMARY OF THE INVENTION

A synthetic jet actuator comprises a first cavity layer comprising aninternal cavity for reception of a fluid volume and an orifice providinga fluid communication between the cavity and an external atmosphere; anda first oscillatory membrane comprising a piezoelectric material adaptedto deflect the first oscillatory membrane in response to an electricalsignal. The cavity has an opening in at least one planar surface of thefirst cavity layer, and the first cavity layer and the first oscillatorymembrane are joined by a high strength, low shear modulus adhesivematerial with the first oscillatory membrane positioned adjacent to theplanar surface having the cavity opening and adapted as an enclosingsurface to said cavity opening. The first oscillatory membrane isadapted to compress and expand a volume within the cavity, based on adeflection generated by the piezoelectric material, for generating afluid flow between the cavity and the external atmosphere through theorifice.

The first oscillatory membrane comprises a substrate that has a highstorage modulus and low loss modulus, and a tan delta of less than 0.5.The piezoelectric material joined to the substrate by a bonding materialhaving a high strength, high shear modulus covalent and cohesive bond.The substrate and the bonding material are electrically conductive, andthe substrate and the bonding material are adapted to act as anelectrical connection for the delivery of an electrical power to thepiezoelectric material, with the electrical connection configured toprovide a non-uniform spatial coverage on the piezoelectric material forproducing an anisotropic deflection of the oscillatory membrane.

The adhesive material joining the oscillatory membrane and the cavitylayer comprises at least one of: an adhesive film positioned between thefirst cavity layer and the first oscillatory membrane, and an adhesiveliquid applied to a surface of at least one of the first cavity layerand the first oscillatory membrane. The first oscillatory membrane ispositioned such that a central axis of the oscillatory membrane alignswith a central axis of the cavity in the first cavity layer, and suchthat a central axis of the piezoelectric material also aligns with thecentral axis of the cavity in the first cavity layer.

The first oscillatory membrane is adapted with a pre-stressed state suchthat in a non-powered state said oscillatory membrane rests at a neutralaxis that provides a slightly expanded state to the cavity of the firstcavity layer, and such that the first oscillatory membrane is forced tobuckle upon deflecting to a compressed state under power of thepiezoelectric material. Preferably, the first oscillatory membrane isadapted with a pre-stressed state of at least 0.01 in.

In some examples, the first cavity layer comprises an expansion chamberembedded within the orifice, the expansion chamber comprising a seriesof baffles for buffering fluid flows that pass through the orifice. Insome examples, a boundary surface of the cavity in the first cavitylayer comprises one or more sloped surfaces having a curvature that ispredetermined to correspond with a curvature of the first oscillatorymembrane in a deflected compression state for minimizing volume withinthe cavity that is predetermined to correspond with stagnant fluid flow.The actuator is configured to generate jet velocities greater than 50m/s at resonance frequencies below 500 Hz; and preferably jet velocitiesin a range of greater than 50 m/s to 100 M/s at resonance frequencies ina range of about 150 Hz to 475 Hz.

In one example, the actuator further comprises a second oscillatorymembrane comprising a piezoelectric material adapted to deflect thesecond oscillatory membrane in response to an electrical signal, withthe cavity of the first cavity layer is formed as a through-hole passingthrough the entire cavity layer, and having two openings at oppositeplanar surfaces of the first cavity layer. The first oscillatorymembrane is positioned adjacent to a first planar surface of the firstcavity layer having a first opening of the cavity and is adapted as anenclosing surface to said first cavity opening, and the secondoscillatory membrane is positioned adjacent to a second planar surfaceof the first cavity layer having a second opening of the cavity and isadapted as an enclosing surface to said second cavity opening. Both thefirst and second oscillatory membranes are adapted to compress andexpand a volume within the cavity, based on deflections generated by therespective piezoelectric materials in the separate oscillatorymembranes, for generating a fluid flow between the cavity and theexternal atmosphere through the orifice. The first and secondoscillatory membranes are both positioned such that central axes of bothrespective oscillatory membranes align with a central axis of the cavityin the first cavity layer, and central axes of the piezoelectricmaterial of both respective oscillatory membranes also align with thecentral axis of the cavity in the first cavity layer.

In another example, the actuator further comprises a second cavity layercomprising an internal cavity for reception of a fluid volume and anorifice providing a fluid communication between the cavity and anexternal atmosphere; a second oscillatory membrane comprising apiezoelectric material adapted to deflect the second oscillatorymembrane in response to an electrical signal; and a third oscillatorymembrane comprising a piezoelectric material adapted to deflect thesecond oscillatory membrane in response to an electrical signal. Thecavities in both the first and second cavity layers are formed asthrough-holes passing through the entirety of the respective cavitylayer, both cavities having two openings at opposite planar surfaces ofthe respective cavity layer. The first oscillatory membrane ispositioned adjacent to a first planar surface of the first cavity layerhaving a first opening of the cavity in the first cavity layer and isadapted as an enclosing surface to said first cavity opening of thecavity in the first cavity layer. The second oscillatory membrane ispositioned adjacent to both a second planar surface of the first cavitylayer having a second opening of the cavity in the first cavity layerand a first planar surface of the second cavity layer having a firstopening of the cavity in the second cavity layer and is adapted as anenclosing surface to both said second cavity opening of the cavity inthe first cavity layer and said first cavity opening of the cavity inthe second cavity layer. The third oscillatory membrane is positionedadjacent to a second planar surface of the second cavity layer having asecond opening of the cavity in the second cavity layer and is adaptedas an enclosing surface to said second cavity opening of the cavity inthe second cavity layer.

In this example, the first and second oscillatory membranes are adaptedto compress and expand a volume within the cavity of the first cavitylayer, based on deflections generated by the respective piezoelectricmaterials in the separate oscillatory membranes, for generating a fluidflow between the cavity and the external atmosphere through the orifice.The second and third oscillatory membranes are adapted to compress andexpand a volume within the cavity of the second cavity layer, based ondeflections generated by the respective piezoelectric materials in theseparate oscillatory membranes, for generating a fluid flow between thecavity and the external atmosphere through the orifice. The secondoscillatory membrane is adapted to expand a volume within the cavity ofthe second cavity layer while concurrently compressing a volume withinthe cavity of the first cavity layer, and to compress a volume withinthe cavity of the second cavity layer while concurrently expanding avolume within the cavity of the first cavity layer.

In another example, the actuator further comprises a second cavity layercomprising an internal cavity for reception of a fluid volume and anorifice providing a fluid communication between the cavity and anexternal atmosphere, with the cavities in both the first and secondcavity layers are formed as blind-holes having only a single opening inone planar surface of the respective cavity layers. The firstoscillatory membrane is positioned between the first and second cavitylayers, adjacent to a planar surface of the first cavity layer havingthe cavity opening of the cavity in the first cavity layer and adjacentto a planar surface of the second cavity layer having the cavity openingof the cavity in the second cavity layer, and is adapted to compress andexpand a volume within the cavity of the first cavity layer, and tocompress and expand a volume within the cavity of the second cavitylayer. The first oscillatory membrane is adapted to expand a volumewithin the cavity of the second cavity layer while concurrentlycompressing a volume within the cavity of the first cavity layer, and tocompress a volume within the cavity of the second cavity layer whileconcurrently expanding a volume within the cavity of the first cavitylayer.

Actuators according to the present invention may also comprise anacoustical enclosure provided outside of the first cavity layer and thefirst oscillatory membrane for containing noise generated by the firstoscillatory membrane. The acoustical enclosure comprises an outer shellwith an absorbent material and an acoustic barrier positioned within theouter shell, the absorbent material being positioned outside of theoscillatory membrane and the acoustic barrier being positioned outsideof the absorbent material.

Actuators according to the present invention may also comprise anacoustic nozzle positioned at an exterior of the orifice of the firstcavity layer, and adapted to extend a flow path for fluid flows passinginto and out from said orifice. The acoustic nozzle comprises anexterior ring made of an acoustic barrier material, with an acousticsubstrate and an acoustic absorbent layer provided within the exteriorring, the acoustic substrate being positioned outside the extended flowpath provided to the orifice of the cavity layer, and the acousticabsorbent layer being positioned outside the acoustic substrate. Theacoustic nozzle may be a monolithically integral component of the cavitylayer, and the flow path provided within the acoustic nozzle comprises aflow expansion chamber.

The present invention is also inclusive of methods of making actuators,comprising steps of forming the oscillatory membrane by joining thepiezoelectric material to a substrate; pre-stressing the oscillatorymembrane during assembly through heat forming via voltage compressionand/or electrically actuating the piezoelectric material; positioningthe first oscillatory membrane adjacent to a planar surface of theplanar surface of the first cavity layer having the cavity opening andjoining the first oscillatory membrane and the cavity layer by anadhesive material; positioning an independent mass structure within theoscillatory membrane, adjacent the piezoelectric material, and securingthe independent mass structure in place with a high strength, low shearmodulus adhesive; generating a vacuum pressure to apply a uniformatmospheric pressure to press the cavity layer and oscillatory membranetogether; and heat curing the cavity layer and oscillatory membranewhile apply the uniform atmospheric pressure.

Both the foregoing general description and the following detaileddescription are exemplary and explanatory only and are intended toprovide further explanation of the invention as claimed. Theaccompanying drawings are included to provide a further understanding ofthe invention; are incorporated in and constitute part of thisspecification; illustrate embodiments of the invention; and, togetherwith the description, serve to explain the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the invention can be ascertained fromthe following detailed description that is provided in connection withthe drawings described below:

FIGS. 1A-1B show schematics of a conventional synthetic jet actuator,with FIG. 1A showing an assembled view of a clamped actuator, and FIG.1B showing an exploded view of the clamped actuator;

FIGS. 2A-2C show schematics of an example of an actuator according tothe present invention, with FIG. 2A showing an assembled view of theactuator; FIG. 2B showing an exploded view of the actuator separatingthe oscillatory membrane layers from the cavity layer; and FIG. 2Cshowing a further exploded view of the actuator separating theindividual layers of the composite oscillatory membranes;

FIG. 3 shows a schematic of an exploded view of an oscillatory membranein FIG. 2C, in connection with a power source;

FIGS. 4A-4B show schematics of a unimorph assembly of an oscillatorymembrane in the actuator of FIGS. 2A-2C, with FIG. 4A showing anassembled view of the unimorph membrane; and FIG. 4B showing an explodedview of the unimorph membrane;

FIGS. 5A-5I3 show schematics of an assembly of the unimorph membrane inFIGS. 4A-4B with a cavity layer, with FIG. 5A showing an assembled viewof the cavity and unimorph membrane layers; and FIG. 5B showing anexploded view of the cavity and unimorph membrane layers;

FIGS. 6A-6B show schematics of the cavity and unimorph membrane layerassembly in FIGS. 5A-5B with an added mass provided to the unimorphmembrane, with FIG. 6A showing an assembled view of the cavity andunimorph membrane layers with the added mass; and FIG. 6B showing anexploded view of the cavity and unimorph membrane layers with the addedmass;

FIG. 7 shows a schematic of an actuator with prestressed compositeoscillatory membranes;

FIG. 8 shows a schematic for assembly of a fully covalently andcohesively bonded actuator as in FIGS. 2A-2C;

FIGS. 9A-9C show schematics for an example of an actuator according tothe present invention having two cavities for noise reduction, with FIG.9A showing an exploded view of the actuator layers, FIG. 9B showing across-sectional view of the assembled actuator, and FIG. 9C showing anoperational schematic showing action of the oscillatory membranes in theactuator;

FIGS. 10A-10C show schematics for another example of an actuatoraccording to the present invention having two cavities for noisereduction, with FIG. 10A showing an exploded view of the actuatorlayers, FIG. 10B showing a cross-sectional view of the assembledactuator, and FIG. 10C showing an operational schematic showing actionof the oscillatory membrane in the actuator;

FIGS. 11A-11C show schematics for an example of an actuator according tothe present invention having a centrally aligned cavity and oscillatorymembranes, with FIG. 11A showing an exploded view of the actuatorlayers, FIG. 11B showing a cross-sectional view of the assembledactuator, and FIG. 11C showing comparative schematics of off-centeredand centered actuator arrangements:

FIGS. 12A-12B show schematics for an example of an actuator according tothe present invention having a composite acoustical enclosure forside-based noise reduction, with FIG. 12A showing an exploded view ofthe actuator layers, and FIG. 12B showing a cross-sectional view of theassembled actuator;

FIGS. 13A-13B show schematics for an example of an actuator according tothe present invention having a noise-reducing acoustic nozzle, with FIG.13A showing an exploded view of the actuator layers, and FIG. 13Bshowing a cross-sectional view of the assembled actuator;

FIGS. 14A-14B show schematics for an example of an actuator according tothe present invention having an integrated acoustic nozzle, with FIG.14A showing an exploded view of the actuator layers, and FIG. 14Bshowing a cross-sectional view of the assembled actuator with across-sectional view of the airflow path of the embedded acousticnozzle;

FIGS. 15A-15B show schematics for an example of an actuator according tothe present invention having an embedded acoustic nozzle, with FIG. 15Ashowing an exploded view of the actuator layers, and FIG. 15B showing across-sectional view of the airflow path of the embedded acousticnozzle;

FIGS. 16A-16B show schematics for an example of an actuator according tothe present invention having a sloped cavity, with FIG. 16A showing anexploded view of the actuator layers, and FIG. 16B showing across-sectional view of the assembled actuator; and

FIG. 17 shows a schematic of an array of synthetic jet actuators adaptedfor noise reduction through phase synchronization.

DETAILED DESCRIPTION OF THE INVENTION

The following disclosure discusses the present invention with referenceto the examples shown in the accompanying drawings, though does notlimit the invention to those examples.

The use of any and all examples, or exemplary language (e.g., “such as”)provided herein is intended merely to better illuminate the inventionand does not pose a limitation on the scope of the invention unlessotherwise claimed. No language in the specification should be construedas indicating any non-claimed element as essential or otherwise criticalto the practice of the invention, unless made clear in context.

As used herein, the singular forms “a,” “an,” and “the” include pluralreferents unless the context clearly dictates otherwise. Unlessindicated otherwise by context, the term “or” is to be understood as aninclusive “or.” Terms such as “first”, “second”, “third”, etc. when usedto describe multiple devices or elements, are so used only to convey therelative actions, positioning and/or functions of the separate devices,and do not necessitate either a specific order for such devices orelements, or any specific quantity or ranking of such devices orelements.

The word “substantially”, as used herein with respect to any property orcircumstance, refers to a degree of deviation that is sufficiently smallso as to not appreciably detract from the identified property orcircumstance. The exact degree of deviation allowable in a givencircumstance will depend on the specific context, as would be understoodby one having ordinary skill in the art.

Use of the terms “about” or “approximately” are intended to describevalues above and/or below a stated value or range, as would beunderstood by one having ordinary skill in the art in the respectivecontext. In some instances, this may encompass values in a range ofapprox. +/−10%; in other instances there may be encompassed values in arange of approx. +/−5%; in yet other instances values in a range ofapprox. +/−2% may be encompassed; and in yet further instances, this mayencompass values in a range of approx. ±/−1%.

It will be understood that the terms “comprises” and/or “comprising,”when used in this specification, specify the presence of statedfeatures, integers, steps, operations, elements, and/or components, butdo not preclude the presence or addition of one or more other features,integers, steps, operations, elements, components, and/or groupsthereof, unless indicated herein or otherwise clearly contradicted bycontext. Recitations of a value range herein, unless indicatedotherwise, serves as a shorthand for referring individually to eachseparate value falling within the stated range, including the endpointsof the range, each separate value within the range, and all intermediateranges subsumed by the overall range, with each incorporated into thespecification as if individually recited herein.

Unless indicated otherwise, or clearly contradicted by context, methodsdescribed herein can be performed with the individual steps executed inany suitable order, including: the precise order disclosed, without anyintermediate steps or with one or more further steps interposed betweenthe disclosed steps; with the disclosed steps performed in an orderother than the exact order disclosed; with one or more steps performedsimultaneously; and with one or more disclosed steps omitted.

FIGS. 1a-1b show one example of a conventional synthetic jet actuator100, in the form of a clamped actuator, both in an assembled state (FIG.1a ) and an exploded state (FIG. 1b ). In this conventional example, theactuator 100 is shown to include several layers, including a centralcavity layer 110; two oscillatory membrane layers 120 a/120 b; and twoouter layers 130 a/130 b. The several layers in the conventionalactuator 100 are held together by a number of clamping elements 140,illustrated in this example by a number of bolts and mating nuts (notshown). Each bolt has a head with a threaded shaft extending therefrom,and the threaded shaft of each bolt is made to extend through acorresponding series of through-holes in the several layers and mated toa respective nut such that the several layers of the actuator 100 arecompressed between the nut and the bolt head.

Conventional actuators, such as the actuator 100, have been found tohave a number of undesirable limitations, including performancelimitations, scaling factor limitations, and integration factorlimitations. Performance issues arise as a result of compromisedrobustness of the several layers in the assembled state, compromisedrepeatability due to uneven pressure distribution within the cavitylayer 110, and inefficiencies due to air leakage of the assembledactuator 100. Scaling factors limitations are realized due to eachactuator requiring a manual assembly, with many components required formechanically fastening the several layers of each actuator. Theseconventional actuators are also limited by integration factors such asthe assembled actuator having an excessive mass that is prohibitive forsmaller applications that require positioning of the actuator in aconfined space (e.g., an airplane wing, a wind turbine blade, or anautomotive body part), due to additional weight that is required byclamping elements and related hardware.

The present invention addresses synthetic jet actuators that employ oneor more piezoelectric-driven oscillatory membranes for generating acompressed jet flow, and is inclusive of actuators that are especiallysuited for integration into aerodynamic surfaces, as required in manycommercial applications, and methods of producing such actuators.Stacked layers in actuators according to the present are secured to oneanother through bonding, such that the resultant actuator is made to belightweight, small in overall size, and easy to produce in cost andenergy efficient processes.

Generally, in a piezoelectric-driven actuator, a voltage is supplied todeflect a piezoelectric along with a corresponding oscillatory membranefor increasing a pressure within a corresponding cavity and forciblyexpelling a fluid from the cavity through a shaped orifice. The presentinvention provides novel material combinations, design features andfabrication techniques to achieve unprecedented performance beyond thatmade available by conventional actuators, and which allow for optimizedperformance parameters that were previously thought mutually exclusivefrom one another, such as aerodynamic performance, acousticalperformance, integration into confined spaces, and use of multiplesimultaneously acoustically-synchronized actuators.

Synthetic jet actuators according to the present invention are capableof generating synthetic jets with reduced sound levels, withoutcompromising aerodynamic performance of the generated jets. Thereduction in sound levels may be achieved in a number of ways, includingthough not limited to changes in the shape of the actuators and theequipment used to operate them, with unprecedented performance in thereduction of noise levels that overcomes previous approaches whileallowing for optimization of further performance parameters.

Systems and methods according to the present invention are proposed forproducing synthetic jet actuators based on piezoelectric actuatedmembranes that overcome limitations of prior conventional systems andmethods by being able to maximize jet velocities and momentums whilereducing resonance frequencies (e.g., below 500 Hz), minimizing theamount of piezoelectric material, minimizing power and energyconsumption, and minimizing overall weight and size of the actuator.

Systems and methods according to the present invention are also proposedfor achieving high jet velocities and momentums, while remainingsuitable for integration into confined spaces (e.g., surface integrationinto commercial applications), and achieving compliance with regulatorystandards such as those prescribed by the US Federal Motor CarrierSafety Administration (FMCSA) and the European Union,

FIGS. 2a-2c show one example of a clampless actuator 200 according tothe present invention, with the actuator 200 show in both an assembledstate (FIG. 2a ) and an exploded state (FIG. 2b ); and with FIG. 2 cshowing further exploded view.

As shown in FIGS. 2a-2b , the actuator 200 comprises a cavity layer 210that is sandwiched between two composite oscillatory membrane layers 220a/220 b. The cavity layer 210 comprises a body having first and secondplanar surfaces, an outer perimeter, and an inner perimeter, the innerperimeter defining a cavity 211 within the cavity layer 210. The cavity211 is open to at least one of the two planar surfaces, and may be opento both planar surfaces. When placed adjacent thereto, an oscillatorymembrane layer 220 serves as an enclosing surface over an opening to thecavity 211, thereby defining an enclosed space within the cavity 211 forretention of a fluid volume. The cavity layer 211 further comprises anorifice 212 that provides an airflow path between the cavity 211 and anatmosphere external to the actuator 200 for the intake and output ofairflows to and from the cavity 211.

As shown in FIG. 2 c, both of the oscillatory membranes 220 a/220 bcomprises a substrate 221 that is integrally coupled to a piezoelectricdisc 223 through a high strength, high shear modulus covalent andcohesive bond 222. The cohesive bond 222 may be formed by a one or twopart thermoset, such as but not limited to a two part epoxy, a one-partmethacrylate, or a low melting metal such as a silver solder. In someexamples, the cohesive bond 222 may be formed through a thermosettingfilm adhesive or “prepreg”. In some examples, as will be discussed infurther detail hereafter, the cohesive bond 222 may be electricallyconductive. Both of the oscillatory membranes 220 a/220 b furtherinclude outer layers 224 that enclose and secure the several layers ofthe respective membrane 220 in place.

In use, the piezoelectric disc 223 is operable through supply of anelectrical power from a. power source 226 (see FIG. 3) via an electricalconnection 225. Supply of an electrical power to the piezoelectric disc223 causes the disc to deform, and because of the integral coupling ofthe disc 223 to the substrate 221, deformation of the disc 223 causes acorresponding deflection of the substrate 221 in the same direction.Preferably, the substrate 221 is formed with a material that issubstantially thin, has a high storage modulus, and has a minimum lossmodulus that results in a minimized tangent of delta (tan delta). Rangesfor these parameters, along with preferred values thereof, are providedin Table I.

In some examples, the substrate 221 may be a metal, such as but notlimited to steels, aluminum, titanium, brass, copper, or the like,including alloys of the same. In some examples, the metal may beselected from grades that are corrosion resistant, such as but notlimited to stainless steels. The metal may also be modified to providefurther corrosion resistance or enhanced bond strengths through the useof surface treatments, such as but not limited to, solvent cleaning,abrasion, acid etching, caustic treatments, electroplating, as well ascombinations of the same. The surface may also be treated with highenergy processes such as flame, corona, plasma, or electrical arc. Inone preferred embodiment, the substrate is 0.005″ 6061 aluminum that hasbeen cleaned with acetone, abraded with 120 grit alumina, and chromicacid etched prior to bonding to the piezoelectric.

In some examples, the substrate 221 may comprise a high modulus polymer,filled polymer, or fiber-reinforced polymer. These polymers may includethermoplastics or thermosets and may comprise solids such as but notlimited to particles, hollow microspheres, nanocomposites, clays,fibers, or flakes, of polymers, metals, or minerals including but notlimited to glass, talc, carbon and graphite. The fibers may be choppedfibers, discontinuous fibers, short or long, or continuous, woven,non-wovens, or random oriented mat. Examples of suitable fibers include,though are not limited to: fiberglass, e-glass, s-glass, cr-glass,carbon fiber, low modulus carbon, medium modulus carbon, high moduluscarbon, polyethylene, polypropylene, nylon, polyester, aramid, Kevlar,PPS, and combinations thereof. The fiber-reinforced matrix may also beproduced from a “pre-preg”.

The substrate 221 may comprise multiple layers, with each of the layerschosen independently from the disclosed materials herein, with thedifferent layers having independent fiber orientations. The fiberlaminate may be balanced or unbalanced, symmetric or asymmetric. Thepolymer matrix may include thermosets or thermoplastics, including butnot limited to epoxies, urethanes, polyesters, vinyl esters, PPS,polypropylene, PTFE, Teflon, polyethylene, or phenolics. The matrix maybe further modified with tougheners, flexibilizers, wetting agents,defoamers, Tg enhancers, materials with densities greater than 2.6g/cm3, or other materials known by those skilled in the art of compositematerials. In one preferred embodiment, the substrate is a preformed0.005″ 0/90 woven laminate of glass in amine-cured epoxy matrix that issolvent cleaned with isopropanol, and abraded with 220 grit sandpaper,and dry wiped prior to bonding the piezoelectric disc.

In some examples, the piezoelectric disc 223 may be covalently bonded tothe substrate 221 during fabrication of the substrate 221, with may beperformed through processes including but not limited to injectionmolding, overmolding, thermoforming, heat sealing, RTM, VARTM, SCRIMP,vacuum bagging, autoclave processing, or wet lay-up. In one preferredembodiment, the piezoelectric disc 223 is placed on a square woven 7 oze-glass mat wet-out with an amine-cured epoxy. Vacuum bagging techniquesmay be used to apply even pressure application during an elevatedtemperature cure during which time the disc 223 is integrally bonded tothe substrate 221 as the substrate is formed.

As shown in FIGS. 2a -2 c, the layered composite oscillatory membranes220 may comprise one or more outer layers 224, positioned outward of thepiezoelectric disc 223. Previously, conventional actuators have used lowmodulus outer layers or no layer at all, as it has been conventionalwisdom that a high modulus layer bonded to the piezoelectric disc, butoff of the neutral bending axis, could not be used as it would preventor inhibit deflection of the membrane. Surprisingly however, it has beenfound that with the present invention, use of a high modulus outer layer224, which may be formed of a material substantially similar to thesubstrate 221, such as a thermoplastic, thermoset, or metal, can lead tounexpected increases in jet velocity. While not being bound by theory,it is believed that coupling of the piezoelectric disc 223 to thesubstrate 221 at the edge of the disc 223 increases the coupling to thesubstrate 221, which also increases deflection of the substrate 221. Inone preferred embodiment, one or more outside layers 224 is provided inthe form of a 7 oz, square-woven, fiberglass in a cycloaliphaticamine-cured epoxy laminate.

FIG. 3 shows an exploded view of a single composite oscillatory membrane220. As illustrated in this example, the piezoelectric disc 223 is madeto deform through supply of an electrical power via the electricalconnection 225, which is illustrated in this example in the form of anelectrode provides an electrical communication between a power source226 and a surface of the piezoelectric disc 223. Suitable examples of anelectrical connection include, through are not limited to: wires,ribbons, tapes of conductive metals, fibers, as well as stenciled, silkscreened, or printed circuits. In some examples, the substrate 221 maycomprise a conductive material such as a metal or carbon fiber thatserves the dual role as an electrode and a mechanical component of thelayered composite membrane.

The electrical connection 225 may connect to the surface of thepiezoelectric disc 223 at any location and over any percentage thereof.In some examples, the electrical connection 225 may connect only to theedge or the middle of the piezoelectric disc 223; in other examples theelectrical connection 225 may be shaped as a ring, or may be spiralshaped to reduce bending strains thereon. The electrical connection 225may further comprise a conductive pressure sensitive adhesive,methacrylate, filled thermoset, solder, or a conductive veil such as ametal-coated nonwoven or wire mesh, or an anisotropic electricallyconductive adhesive film (ACF). In a preferred embodiment, theelectrical connection 225 comprises a ⅛″ wide copper tape with anelectrically conductive pressure sensitive adhesive (PSA). In anotherpreferred embodiment, the electrical connection 225 comprises a Ni orCu-coated carbon veil having an aerial weight of 20 g/m2, such as thosesupplied by Technical Fibre Products, The surface of the piezoelectricdisc 223 may also comprise a conductive layer that may be thin with alow bending stiffness, such as but not limited to nickel plating orcopper cladding. In some examples, the piezoelectric disc 223 may have anickel surface deposited at 20 micron thickness.

In one preferred embodiment, the electrical connection 225 may bedesigned so as to intentionally provide non-uniform spatial coverage onthe piezoelectric disc 223. The non-uniform coverage, combined withelectrical resistance from the surface of the piezoelectric disc 223,produces an anisotropic deflection of the oscillatory membrane 220 thatis non-uniform in both time and position. By careful selection of theorientation of the electrical connection 225 and the surface resistanceof the piezoelectric disc 223, unexpected improvements in jet velocitymay be observed. In a preferred embodiment, a ⅛″ copper tape, runningperpendicular to the orientation of the orifice 212, is connected to theNi-sputtered surface of the piezoelectric disc 223 using a conductivePSA, wherein the PSA has a resistance of less than 2 ohm across theconnection, but the surface of the electrode has a resistance of 10-40ohm per inch away from the copper ribbon. Supply of power to the coppertape causes deflection to originate along the central axis of thepiezoelectric disc 223 with the highest amplitude, though, due toresistive losses across the surface of the disc 223, the amplitudedecreases as it approaches an edge thereof. This induces a partial waveshape to form in the piezoelectric disc 223, and the oscillatorymembrane 220 as whole, thereby directing a fluid in the cavity 211towards the orifice 212 and generating jet velocity increases of up to5%. In addition, the reduction in the actuation of the piezoelectricdisc 223 near the periphery thereof reduces the strain in the transitionof the materials, which has been found to result in significantimprovements to the longevity of the disc 223.

Piezoelectric materials are well known to those skilled in the art andmay be selected from a group including but not limited to PZT, quartz,zinc oxide. PVDF, barium titanate, or the like. Because these materialsare relatively expensive, there is interest in limiting the amount ofmaterial used in the oscillatory membrane 220. In some examples, areduction in piezoelectric material may be achieved through use ofrelatively thin piezoelectric discs 223 between 0.01 and 0.1″ thickness,with inclusion of only one piezoelectric disc 223 per oscillatorymembrane 220. An oscillatory membrane using only a single piezoelectricdisc 223 may be referred to as a unimorph. FIGS. 4a-4b show one exampleof a unimorph membrane 220′ for ease of illustration, the illustratedexample foregoes depiction of the adhesive bond 222 and outer layers224.

By using only a single piezoelectric disc 223, a unimorph membrane 220′has an advantage of reduced power and energy consumption, as well as alower resonance frequency. In a unimorph membrane 220′, thepiezoelectric disc 223 may be placed on the neutral bending axis of theoscillatory membrane, as shown in FIGS. 4a -4 b, inside or outside thecavity 211 of the cavity layer 210. In a preferred embodiment, theunimorph membrane 220′ is designed such that the single piezoelectricdisc 223 is on the side of the substrate 221 opposite the cavity 211.FIGS. 5a-5b show one such example of a unimorph membrane 220′ asassembled in an actuator 200 that employs only a single oscillatorymembrane 220, with the piezoelectric disc 223 of the single unimorphmembrane 220′ positioned on an outside surface of the substrate 221,opposite the cavity 211. Again, for ease of illustration, theillustrated example foregoes depiction of the adhesive bond 222 andouter layers 224.

In some examples, deflection and resonance frequency of the oscillatorymembrane 220 may be further turned through the use of high densitymaterials. Through empirical study, it was discovered that increasingthe mass of the oscillatory membrane 220, without significantly changingother mechanical properties, can dramatically change the response of themembrane 220, Such an increase in mass may be achieved by inlaying highdensity materials, such as tungsten, into the substrate 221, or byforming the substrate 221 substantially of tungsten. In other examples,the added mass may be achieved by filling the thermoplastic andthermoset of the composite oscillatory membrane 220 with high densitymaterials, such as barium sulfate. In some examples, an additionalindependent mass structure may be incorporated into the compositeoscillatory membrane 220, FIGS. 6a-6b illustrate one example of anactuator 200 in which a 5 oz lead disc 227 is attached to the center ofthe outside surface of the piezoelectric disc 223 with a high strength,low shear modulus adhesive (not shown). Because of the location ofattachment and low shear modulus of the adhesive, the piezoelectric disc223 is still allowed to bend freely, while the added mass 227 decreasesresonance frequency by 40% without significantly affecting jet velocity.

While not being bound by theory, it is believed that an added massincreases the momentum of the oscillatory membrane 220, causing greaterdisplacement as it is cyclically powered. In addition, the increase inweight causes both a decrease and broadening of the resonance frequency.This is especially beneficial in the use of net zero mass flow clamplessactuators in aerodynamics applications, including tractor trailertrucks, as the market requires a resonance frequency of less than 500Hz. By increasing the mass of the oscillatory membrane 220, it ispossible to decrease and broaden the resonance frequency from 800 Hzdown to frequencies within a range of about 150 to about 475 Hz.

Buckling actuators are well known to those skilled in the art. Indesigns according to the present invention, a non-powered oscillatorymembrane 220 rests at a neutral bending axis that results in a slightexpanded state as illustrated in FIG. 7. When powered, the oscillatorymembrane 220 is forced to buckle and snap to an opposite side from theneutral bending axis, which is expected to reduce resonance frequencywhile increasing the force in which the membrane acts against the airinside the cavity, resulting in higher jet velocity values (compared toa non-buckling actuator of similar size) at lower sound level (reductionin actuation frequency equates to lower sound levels).

As an alternative to conventional mechanical displacement as a means forpre-stressing, the present invention is inclusive of a voltagecompression may be used to displace the oscillatory membrane 220 throughheat forming by raising a temperature of the membrane 220 above itssoftening or melting point, deforming it, and then cooling it in thedeformed state. Heat forming may be done before or after assembly of theactuator. In some examples, an actuator may be made with a variabledisplacement by assembling the actuator using mechanically fastenedplates, such that the fasteners may enable the membrane to be repeatedlydisplaced or repositioned, and then clamped, in order to fine tuneperformance metrics.

Buckle geometry of the actuator 200 may be optimized by electricallyactuating the piezoelectric disc 223 during assembly of the actuator. Anactuated piezoelectric disc 232 will deform to its desired geometry,thus by actuating the piezoelectric disc 223 during assembly of theactuator the assembled actuator 200 will be pre-optimized forcombination of the specific piezoelectric disc 223 and substrate 221.Control of the static voltage can further fine tune the buckled shapeand performance parameters.

The present invention is inclusive of methods of fabricating oscillatorymembranes 220, as well as actuators 200 that incorporate such membranes,to produce synthetic jet actuators that have fully cohesively andcovalently bonded structures with uniform bond strength and minimizedinternal stresses.

Prior approaches to assemble zero net mass flow clamped edgepiezoelectric-driven synthetic jet actuators have relied on mechanicalclamping. Those conventional approaches have a downside of increasingthe number of parts and assembly steps, presenting a possibility ofleaks through interfaces, as well as increased mass and volume, and thefurther potential for mechanical failure due to the loosening offasteners overtime while the actuator is in service. Non-mechanicalapproaches that have been explored include injection molding,over-molding, or hot pressing. While these techniques boast highthroughput, the use of high thermal stresses and non-uniform pressureshave been found to result in actuators with in-situ strains and poorrepeatability. Other approaches have included the use of liquidadhesives, though these have been found to not allow for tight controlof bond lines and present a risk fouling in small design features, suchas the orifice 212 in the cavity layer 210.

The present invention avoids the downsides of the prior approaches byinstead adopting a novel use of a B-staged thermosetting adhesive filmand vacuum bag processing to produce actuators with tight control ofbond lines and spatially even inter-laminar bond strengths that improvejet performance, reduce complexity, lower overall weight, and improvepart-to-part consistency.

Assembly of one or more oscillatory membranes 220 to a cavity layer 210may include surface preparation of the layer faces, as previouslydescribed. In one embodiment, a B-staged thermosetting adhesive film iscut or punched from sheet stock and placed between the membrane andcavity layers 220/210. The film adhesive may also be applied as a liquidto either one or both layers, and B-staged prior to assembly. The filmadhesive may also be provide in the same form as the cohesive/adhesivebond 222. As shown in FIG. 8, the several layers are then sealed in abag 30 that is connected to a vacuum source 40 that removes air from thebag 30 to thereby apply an atmospheric pressure evenly over the surfaceof the layered components. As opposed to prior approaches that use hotpressing, which produces point loads on small features andmisalignments, use of a vacuum-based atmospheric pressure assures aperfectly uniform lamination of the layers and greatly reduces capitalcosts by not requiring expensive molds. Numerous vacuum bags 30 may beassembled and loaded into a process oven where the temperature israised, causing the film adhesive to briefly reflow and form cohesivecovalent bonds between the layered components before curing.

The layered components may be cured for a period of about 1 hour toabout 4 hours, at a temperature in a range of about 140° F. to about350° F, for the thermoset to reach full properties. In a multi-stepassembly, the maximum cure temperature may be saved for the final stepin order to “co-cure” all the layers of the composite actuator.Examination of bond lines after such curing has been found to yieldcontrol of the adhesive flow to within a 0.030″ precision, It has alsobeen found that actuators fabricated under such conditions are capableof increased jet velocities that are up to 25% greater than thosegenerated by mechanical fastened actuators.

While not being bound by theory, it is believed that methods accordingto the present invention, with a high modulus bond greater than about8,000 PSI, provide a greater and more spatially uniform interface of theoscillatory membranes to the cavity than is achieved with mechanicalfasteners, while also eliminating leak paths. The inventive methods alsoprovide significant weight, cost, and parts savings by not requiringoutside clamps, allowing these net zero mass flow clampless actuators tobe used in practical applications including improving aerodynamics oftractor trailer trucks.

Certain materials, especially those which are corrosion resistant andintrinsically unreactive, may be difficult to covalently bond withthermosets. These may include materials such as stainless steels,aluminum, or polypropylene. If desired, a “tie-layer” may be used inplace of the thermoset to achieve similar results. A “tie layer” isusually one or a combination of two or more mutually compatiblematerials that form a bonding layer between two mutually incompatiblematerials. Tie layers may include, for example, a thermoplastic materialthat provides adhesion to two adjacent materials, most often throughmelt processing or chemical reactions; modified acrylic acid, oranhydride grafted polymers or those similar to but not limited toDuPont's Bynel, Nucrel, and Fusabond grades, or those described andreferenced, as further examples, in U.S. Pat. Nos. 8,076,000; 7,807,013;and 7,285,333. The melting point or melt index of the tie layer may beselected so that the tie-layer can be post-processed withoutsubstantially melting or flowing other non-metallics in the rest of theactuator.

Methods according to the present invention, including the use of one ormore of non-uniform electrodes, high modulus outer layers,covalent-cohesive bonding, vacuum bagging, and careful control ofsubstrate thickness, storage, and loss moduli, as well as massdistribution, are effective for producing synthetic jet actuators thatare capable of overcoming the limitations of prior designs, includingthe generation of jet velocities greater than 100 m/s at frequenciesunder 500 Hz, with size and weights that are viable for practicalapplications, as detailed in Table I below,

Parameter Units Range Preferred Substrate Storage Modulus KSI 10-300001000 Substrate Tan Delta E′E″ <5 0.5 Substrate Thickness Inch0.001-0.060 0.005 Piezoelectric Thickness Inch 0.002-0.200 0.010 Numberof Piezoelectrics Per Membrane Each 1-4 2 Piezo Surface Resistivity Ohm1-100 20 Number of Membranes Per Orifice Each 1-2 1.5 Number of OrificesPer Actuator Each 1-2 2 Mass of Added Weight g 1-500 15 (MembraneExcluding Substrate, Electrode, and Disc) Actuator Cross-Sectional Areain2 1-16 10 Actuator thickness in 0.1-1 0.25 Actuator Weight g 10-500 30g Jet Velocity m/s >50 100 Resonance Frequency Hz 1-500 200 ConsumedPower W 0.1-40 5 Prestressed Displacement Inch 0-0.06 0.010 Soundpressure level at 1 m away dBA 10-50 20 from actuator

The present invention is inclusive of additional designs for syntheticjet actuators, including designs that incorporate multiple cavity layerswith one or more oscillatory membrane arrangements (which may includeunimorph membranes), including arrangements in which a commonoscillatory membrane is shared between two cavity layers, as well asarrangements that include additional noise reduction elements

FIGS. 9a-9c show one example of a synthetic actuator 300 composed of alaminated stack of layers that comprises three oscillatory membranes 320a/320 b/320c and two cavity layers 310 a/310 b, with each cavity layer310 having a separate cavity 311 and orifice 312. As shown in FIG. 9 c,the several layers are stacked such that the two cavity layers 310 a/310b cyclically share a common oscillatory membrane 320 b that switchesbetween a first mode in which it compresses a volume in the top cavity311 a and expands a volume in the bottom cavity 311 b, and a second modein which it expands a volume in the top cavity 311 a and compresses avolume in the bottom cavity 311 b. In this way, the actuator 300 isadapted such that airflows at the two orifices 312 a/312 are always inopposite directions to one another. As one skilled in the art willunderstand, a noise level generated by a synthetic actuator is afunction of soundwaves generated by airflows that travel out from andinto an orifice of the actuator. With the arrangement shown in FIG. 9 c,because the airflows are travelling in opposite directions to oneanother, sound waves generated by the individual airflows will beinverted relative to one another such that there is an interferencegenerated therebetween, resulting in a partial cancellation of the soundwaves and thus a reduction in overall noise levels. Acoustic testing ofan actuator with a three oscillatory membrane design as shown in FIGS.9a-9c was found to result in a 14.7 dB reduction in sound power level ascompared to that produced by an individual synthetic jet actuator.Advantageously, as both orifices 312 a/312 b face in a common direction,the reduction in noise level is achieved with insubstantial loss toaerodynamic performance. Additional parameters of a preferred embodimentof the actuator 300 are provided below in Table II.

FIGS. 10a-10c show another example of a synthetic actuator 400 composedof a laminated stack of layers that comprises two capped cavity layers410 a/410 b each having a cavity 411 a/411 b and an orifice 412 a/412 b,and a middle piezoelectric oscillatory membrane 420. As shown in FIG. 10c, the actuator 400 is adapted for the two cavity layers 411 a/411 b tocyclically share the common oscillatory membrane 420 such that airflowsat the two orifices 412 a/412 b are always in opposite directions to oneanother, and such that there is an interference between sound wavesgenerated by the two airflows that results in noise reduction. In thisexample, use of a single oscillatory membrane 420 reduces cost and powerconsumption. Additional parameters of a preferred embodiment of theactuator 400 are provided below in Table II.

FIGS. 11a-11c show a further example of a synthetic actuator 500composed of a laminated stack of layers that comprises two oscillatorymembranes 520 a/520 b and a single cavity layer 510 having a cavity 511and an orifice 512 As opposed to a mechanically fastened actuator, inwhich fastening elements (e.g., screws, bolts, etc.) are located aroundthe cavity 511 and limit the cavity volume, use of lamination in theactuator 500 enables a significantly larger cavity volume as well as analignment between a central axis of the oscillatory membranes and acentral axis of the cavity 511. As seen in the comparison shown in FIG.11 c, whereas a conventional clamped actuator 100 will have a cavity 111with a central axis that is offset from a central axis of theoscillatory membranes 120 a/120 b, an actuator 500 according to thepresent invention is provided with additional space for centralpositioning of the cavity 511 such that a central axis of the cavitywill align with a central axis of the oscillatory membranes 520 a 1520b. Because piezoelectric discs are positioned at centers of theoscillatory membranes 520 a/520 b, alignment of the cavity central axisand the membrane central axis will also result in a concurrentlyalignment with the piezoelectric discs. This alignment of the cavity 511with a larger volume with the central axis of the oscillatory membranes520 a/520 b and the centered piezoelectric discs enables a largerdisplacement of the membranes 520 a/520 b, which results in a greaterjet volume as well as a reduced resonance frequency. Additionalparameters of a preferred embodiment of the actuator 500 are providedbelow in Table II.

FIGS. 12a-12b show a further example of an actuator 600 that is composedof a laminated stack that comprises two oscillatory membranes 61.0 a/610b and one cavity layer 610 having a cavity 611 and an orifice 612. Oneach side of the actuator 600, an acoustical enclosure 630 is providedfor containing noise generated by each oscillatory membrane 620. Theenclosure 630 has a minimum volume that avoids inhibiting performance ofthe actuator 600. Generally, as the frequency decreases by a ⅓ octavethe enclosure volume is doubled. The volume of the enclosure 630 on eachside of the actuator 600 sealed section is about 40 times the volume ofair moved by each piezoelectric disc during actuation. This volume isfilled with an acoustic absorbent material 631 such as melamine foam,fiberglass insulation, or mineral wool material to increase the apparentvolume and lower the resonant frequency. Surrounding the absorbentmaterial 631 is an acoustic barrier 632 such as mass loaded vinyl. Bothmaterials are enclosed in an outer shell 633. These materials havetransmission loss (TL) values of about 24 at 360 Hz and about 29 at 675Hz. Additional parameters of a preferred embodiment of the actuator 600are provided below in Table II.

FIGS. 13a-13b show a further example of an actuator 700 that is composedof a laminated stack that comprises two oscillatory membranes 720 a/720b and one cavity layer 710 having a cavity 711 and an orifice 712. Anacoustic nozzle 740 attached to the exterior plane of the orifice 712provides an extended airflow path in and out from the cavity 711, Theacoustic nozzle 740 is adapted for reducing acoustic levels by providingan airflow path 741 that serves as an extension to the cavity 712, withan interior of the airflow path 741 being formed with an acousticabsorbent layer 742 and an acoustic substrate 743 that are made frommaterial having highly absorptive character at one or more frequenciesof acoustic of interest. Suitable examples of a highly absorptiveacoustic material include, though are not limited to melamine, andacoustic foam. The acoustic nozzle 740 further includes an exterior ring744 made of an acoustic barrier material for further attenuation ofacoustic levels. Additional parameters of a preferred embodiment of theactuator 700 are provided below in Table II.

FIGS. 14a-14b show a further example of an actuator 800 that iscomprised of a laminated stack that comprises two oscillatory membranes820 a/820 b and one cavity layer 810 having a cavity 811 and an orifice812. An acoustic nozzle 840 extends from an exterior plane of theorifice 812 and provides an extended airflow path 841 into and out fromthe cavity 811. In this example, the airflow path 841 of the acousticnozzle 840 is formed with an expansion chamber 842 for attenuatingacoustic levels. Additional parameters of a preferred embodiment of theactuator 800 are provided below in Table II.

FIGS. 15a-15b show a further example of an actuator 900 that is composedof a laminated stack that comprises two oscillatory membranes 920 a/920b and one cavity layer 910 having a cavity 911 and an orifice 912. FIG.15a shows the actuator 900 in an exploded view with the cavity layer 910separated into two halves 910-1/910-2, and the orifice 912 alsoseparated into two halves 912-1/912-2. In this example, the cavity layer910 is provided with an integrated expansion chamber 940 that includes abaffle arrangement 941 that is embedded into the orifice 912, throughformation of an etched pattern in both orifices halves 912-1/912-2 ofFIG. 15 a. FIG. 15b show a partial close-up view of the orifice half912-2 showing the baffle arrangement etched therein. The bafflearrangement 941 provides a buffering effect to an airflow passingthrough the orifice 912, which effectively reduces acoustic levelsgenerated by the airflow. Additional parameters of a preferredembodiment of the actuator 900 are provided below in Table II.

FIGS. 16a-16b show a further example of an actuator 1000 that iscomposed of a laminated stack that comprises two oscillatory membranes1020 a/1020 b and one cavity layer 1010 having a sloped cavity 1011 andan orifice 1012. FIG. 16a omits an illustration of the top oscillatorymembrane 1020 a to provide a fuller view of the cavity 1011. In thisexample, a boundary surface forming the open volume of the cavity 1011at a side opposite the orifice 1012 is formed with sloped surfaces 1013that eliminate dead space and enhance internal pressure distributionwithin the cavity 1011. The sloped surface 1013 optimizes the shape ofthe cavity 1011 to further correspond with properties of the oscillatorymembranes 1020 a/1020 b and the actuation frequency. In particular, thesloped surface 1013 is formed with opposite surfaces having curvaturesthat correspond to maximum curvatures incurred in the membranes 1020a/1020 b when those membranes are at a maximum inward displacementtoward the cavity 1011, though with a slight offset provided to thesloped surfaces such that the membranes do not contact the surfaces,thereby avoiding friction that may cause mechanical and thermal wear tothe membranes. This construction of the sloped surfaces 1013 decreases avolume of the internal cavity 1011 by removing portions that wouldnormally retain pockets of stale air that would not be compressed into ajet stream, thereby enhancing efficiency of the actuator, and reducingnoise levels and power consumption. Additional parameters of a preferredembodiment of the actuator 1000 are provided below in Table II.

FIG. 17 shows an example of a system 1 comprising of an array formed ofa plurality of actuators 200 that are each mounted in a common carrierplatform (e.g., a surface mounting for commercial applications), andwhich are each operated by a common arrangement of a multi-channeldriver 10, a controller 20, and one or more sensors 30. An AC voltage issupplied to each actuator 200 with individual phase adjustment forcontrolling the sound levels generated at a. distance greater than thatof two wavelengths, where the wave is effectively a plane wave. Phasecontrol can be used to aim the soundwaves produced by each linear columnof actuators 200, with the degree of acoustic benefit being adjustabledepending on the phase coherence of the system 1. If each driver has adifferent phase response, then the acoustic benefit will not be as greatas a system 1 with similarly phased drivers. By basic calculation, at adistance equivalent to two wavelengths, and with perfect phasing, thesystem 1 may be controlled to produce a zero acoustic level. Achievingsuch an ideal result is difficult in reality, due to challenges inaligning drivers, and further due to inconsistencies in drivermanufacturing that result in phase differences. In examples according tothe present invention, the phase difference between neighboringactuators 200 is adjusted based on noise levels measured while a driveris scanning different phase angles for each actuator 200 on two separatechannels. While the system 1 is discussed in this example as comprisingmultiple actuators 200, it will be understood that the system 1 mayinclude multiple actuators of any type discussed herein, including acombination of multiple different actuator types. Additional parametersof a preferred embodiment of the system 1 are provided below in TableII.

Systems and methods according to the present invention, includingsystems that adopt one or more of the foregoing examples, as well assystems that combine elements of two or more of the foregoing examples,are effective for producing synthetic jet actuators that are capable ofovercoming the limitations of prior designs, including the generation ofjet velocities with reduced noise levels below that which was previouslythought possible, as further detailed in Table II below.

TABLE II Parameter Units Range Preferred Orifice sectional area. in²0.01-0.1 .027 Cavity diameter Inch 1-5 3.125 Cavity volume in³ 0.5-3 0.9Barrier material Weight Lb/sq.ft 0.5-20 2 Barrier material TL@125 Hz dB1-25 21 Barrier material TL@250 Hz dB 130 22 Barrier material TL@325 HzdB 1-30 23 Barrier material TL@500 Hz dB 1-35 27 Barrier material TL@675Hz dB 1-35 29 Barrier material TL@1000 Hz dB 1-35 33 Acoustical volumeto be filled in³ 1-30 1 with absorbent material Piezoelectric ThicknessInch 0.002-0.200 0.010 Number of Piezoelectrics Per Each 1-4 7 MembraneNumber of actuators in a Each 2-60 10 synchronized-phase array ActuatorCross-Sectional Area in² 1-16 10 Actuator thickness In 0.1-1 0.25Actuator Weight G 10-500 30 g Jet Velocity m/s +2250 100 ResonanceFrequency Hz 1-675 200 Consumed Power W 0.1-4 5 Sloped cavity lengthInch 0-1 0.25 Sound pressure level at lm dBA 10-50 20 away from actuator

Although the present invention is described with reference to particularembodiments, it will be understood to those skilled in the art that theforegoing disclosure addresses exemplary embodiments only; that thescope of the invention is not limited to the disclosed embodiments; andthat the scope of the invention may encompass additional embodimentsembracing various changes and modifications relative to the examplesdisclosed herein without departing from the scope of the invention asdefined in the appended claims and equivalents thereto.

While a number of the foregoing examples forego illustration and/ordiscussion of certain elements that are discussed in other examples, itwill be understood that each example may include or be made to includeelements from one or more other examples. For instance, while someexamples forego illustration and/or explicit discussion of a cohesivebond layer and/or an outer layer (such as cohesive bond 222 and outerlayers 224 in FIGS. 2c and 3) it will be understood that each examplediscussed herein may include these elements. Likewise, while someexamples discuss configurations with only a single cavity layer, with asingle cavity and orifice, it will be understood that each examplediscussed herein is also contemplated as comprising multiple cavitylayers, each with a respective cavity and orifice (e.g., examples inFIGS. 10-16, which are each illustrated with single cavity layers, mayeach comprise multiple cavity layers such as is illustrated in theexample of FIG. 9). Furthermore, though the foregoing examples arediscussed relative to an airflow, it will be understood that jetactuators according to the present invention are not limited to air andthe generation of an airflow, and may be used with any suitable fluidfor the generation of a corresponding fluid flow.

To the extent necessary to understand or complete the disclosure of thepresent invention, all publications, patents, and patent applicationsmentioned herein are expressly incorporated by reference herein to thesame extent as though each were individually so incorporated. Nolicense, express or implied, is granted to any patent incorporatedherein.

The present invention is not limited to the exemplary embodimentsillustrated herein, but is instead characterized by the appended claims,which in no way limit the scope of the disclosure.

What is claimed is:
 1. A synthetic jet actuator comprising: a firstcavity layer comprising an internal cavity for reception of a fluidvolume and an orifice providing a fluid communication between the cavityand an external atmosphere; and a first oscillatory membrane comprisinga piezoelectric material adapted to deflect the first oscillatorymembrane in response to an electrical signal, wherein the cavity has anopening in at least one planar surface of the first cavity layer, andthe first oscillatory membrane is positioned adjacent to the planarsurface having the cavity opening and adapted as an enclosing surface tosaid cavity opening, the first oscillatory membrane is adapted tocompress and expand a volume within the cavity, based on a deflectiongenerated by the piezoelectric material, for generating a fluid flowbetween the cavity and the external atmosphere through the orifice, andthe first cavity layer and the first oscillatory membrane are joined bya high strength, low shear modulus adhesive material.
 2. The syntheticjet actuator of claim 1, wherein the adhesive material comprises atleast one of: an adhesive film positioned between the first cavity layerand the first oscillatory membrane, and an adhesive liquid applied to asurface of at least one of the first cavity layer and the firstoscillatory membrane.
 3. The synthetic jet actuator of claim 1, wherein:the first oscillatory membrane comprises a substrate, with thepiezoelectric material joined to the substrate by a bonding material. 4.The synthetic jet actuator of claim 3, wherein: the bonding material isa high strength, high shear modulus covalent and cohesive bond.
 5. Thesynthetic jet actuator of claim 3, wherein: the substrate and thebonding material are electrically conductive, and the substrate and thebonding material are adapted to act as an electrical connection for thedelivery of an electrical power to the piezoelectric material.
 6. Thesynthetic jet actuator of claim 5, wherein: the electrical connection isconfigured to provide a non-uniform spatial coverage on thepiezoelectric material for producing an anisotropic deflection of theoscillatory membrane.
 7. The synthetic jet actuator of claim 3, wherein:the substrate has a high storage modulus and low loss modulus, and a tandelta of less than 0.5.
 8. The synthetic jet actuator of claim 1,wherein the first oscillatory membrane is positioned such that a centralaxis of the oscillatory membrane aligns with a central axis of thecavity in the first cavity layer.
 9. The synthetic jet actuator of claim8, wherein a central axis of the piezoelectric material aligns with thecentral axis of the cavity in the first cavity layer.
 10. The syntheticjet actuator of claim 1, wherein: the first cavity layer comprises anexpansion chamber embedded within the orifice, the expansion chambercomprising a series of baffles for buffering fluid flows that passthrough the orifice.
 11. The synthetic jet actuator of claim 1, wherein:a boundary surface of the cavity in the first cavity layer comprises oneor more sloped surfaces having a curvature that is predetermined tocorrespond with a curvature of the first oscillatory membrane in adeflected compression state for minimizing volume within the cavity thatis predetermined to correspond with stagnant fluid flow.
 12. Thesynthetic jet actuator of claim 1, wherein: the first oscillatorymembrane is adapted with a pre-stressed state such that in a non-poweredstate said oscillatory membrane rests at a neutral axis that provides aslightly expanded state to the cavity of the first cavity layer, andsuch that the first oscillatory membrane is forced to buckle upondeflecting to a compressed state under power of the piezoelectricmaterial.
 13. The synthetic jet actuator of claim 12, wherein: the firstoscillatory membrane is adapted with a pre-stressed state of at least0.01 in.
 14. The synthetic jet actuator of claim 1, wherein: theactuator is configured to generate jet velocities greater than 50 m/s atresonance frequencies below 500 Hz.
 15. The synthetic jet actuator ofclaim 14, wherein: the actuator is configured to generate jet velocitiesin a range of greater than 50 m/s to 100 M/s at resonance frequencies ina range of about 150 Hz to 475 Hz.
 16. The synthetic jet actuator ofclaim 1, further comprising: a second oscillatory membrane comprising apiezoelectric material adapted to deflect the second oscillatorymembrane in response to an electrical signal, wherein the cavity of thefirst cavity layer is formed as a through-hole passing through theentire cavity layer, the cavity having two openings at opposite planarsurfaces of the first cavity layer, the first oscillatory membrane ispositioned adjacent to a first planar surface of the first cavity layerhaving a first opening of the cavity and is adapted as an enclosingsurface to said first cavity opening, and the second oscillatorymembrane is positioned adjacent to a second planar surface of the firstcavity layer having a second opening of the cavity and is adapted as anenclosing surface to said second cavity opening, and both the first andsecond oscillatory membranes are adapted to compress and expand a volumewithin the cavity, based on deflections generated by the respectivepiezoelectric materials in the separate oscillatory membranes, forgenerating a fluid flow between the cavity and the external atmospherethrough the orifice.
 17. The synthetic jet actuator of claim 16, whereinthe first and second oscillatory membranes are both positioned such thatcentral axes of both respective oscillatory membranes align with acentral axis of the cavity in the first cavity layer.
 18. The syntheticjet actuator of claim 17, wherein central axes of the piezoelectricmaterial of both respective oscillatory membranes align with the centralaxis of the cavity in the first cavity layer.
 19. The synthetic jetactuator of claim 1, further comprising: a second cavity layercomprising an internal cavity for reception of a fluid volume and anorifice providing a fluid communication between the cavity and anexternal atmosphere; a second oscillatory membrane comprising apiezoelectric material adapted to deflect the second oscillatorymembrane in response to an electrical signal; and a third oscillatorymembrane comprising a piezoelectric material adapted to deflect thesecond oscillatory membrane in response to an electrical signal, whereinthe cavities in both the first and second cavity layers are formed asthrough-holes passing through the entirety of the respective cavitylayer, both cavities having two openings at opposite planar surfaces ofthe respective cavity layer, the first oscillatory membrane ispositioned adjacent to a first planar surface of the first cavity layerhaving a first opening of the cavity in the first cavity layer and isadapted as an enclosing surface to said first cavity opening of thecavity in the first cavity layer, the second oscillatory membrane ispositioned adjacent to both a second planar surface of the first cavitylayer having a second opening of the cavity in the first cavity layerand a first planar surface of the second cavity layer having a firstopening of the cavity in the second cavity layer and is adapted as anenclosing surface to both said second cavity opening of the cavity inthe first cavity layer and said first cavity opening of the cavity inthe second cavity layer, and the third oscillatory membrane ispositioned adjacent to a second planar surface of the second cavitylayer having a second opening of the cavity in the second cavity layerand is adapted as an enclosing surface to said second cavity opening ofthe cavity in the second cavity layer, both the first and secondoscillatory membranes are adapted to compress and expand a volume withinthe cavity of the first cavity layer, based on deflections generated bythe respective piezoelectric materials in the separate oscillatorymembranes, for generating a fluid flow between the cavity and theexternal atmosphere through the orifice, and both the second and thirdoscillatory membranes are adapted to compress and expand a. volumewithin the cavity of the second cavity layer, based on deflectionsgenerated by the respective piezoelectric materials in the separateoscillatory membranes, for generating a fluid flow between the cavityand the external atmosphere through the orifice.
 20. The synthetic jetactuator of claim 19, wherein the second oscillatory membrane is adaptedto expand a volume within the cavity of the second cavity layer whileconcurrently compressing a volume within the cavity of the first cavitylayer, and to compress a volume within the cavity of the second cavitylayer while concurrently expanding a volume within the cavity of thefirst cavity layer.
 21. The synthetic jet actuator of claim 1, furthercomprising: a second cavity layer comprising an internal cavity forreception of a fluid volume and an orifice providing a fluidcommunication between the cavity and an external atmosphere, wherein thecavities in both the first and second cavity layers are formed asblind-holes having only a. single opening in one planar surface of therespective cavity layers, the first oscillatory membrane is positionedbetween the first and second cavity layers, adjacent to a planar surfaceof the first cavity layer having the cavity opening of the cavity in thefirst cavity layer and adjacent to a planar surface of the second cavitylayer having the cavity opening of the cavity in the second cavitylayer, the first oscillatory membrane is adapted to compress and expanda volume within the cavity of the first cavity layer, and to compressand expand a volume within the cavity of the second cavity layer. 22.The synthetic jet actuator of claim 21, wherein the first oscillatorymembrane is adapted to expand a volume within the cavity of the secondcavity layer while concurrently compressing a volume within the cavityof the first cavity layer, and to compress a volume within the cavity ofthe second cavity layer while concurrently expanding a volume within thecavity of the first cavity layer.
 23. The synthetic jet actuator ofclaim I, further comprising: an acoustical enclosure provided outside ofthe first cavity layer and the first oscillatory membrane for containingnoise generated by the first oscillatory membrane.
 24. The synthetic jetactuator of claim 23, wherein: the acoustical enclosure comprises anouter shell with an absorbent material and an acoustic barrierpositioned within the outer shell, the absorbent material beingpositioned outside of the oscillatory membrane and the acoustic barrierbeing positioned outside of the absorbent material.
 25. The syntheticjet actuator of claim 1, further comprising: an acoustic nozzlepositioned at an exterior of the orifice of the first cavity layer, andadapted to extend a flow path for fluid flows passing into and out fromsaid orifice.
 26. The synthetic jet actuator of claim 25, wherein: theacoustic nozzle comprises an exterior ring made of an acoustic barriermaterial, with an acoustic substrate and an acoustic absorbent layerprovided within the exterior ring, the acoustic substrate beingpositioned outside the extended flow path provided to the orifice of thecavity layer, and the acoustic absorbent layer being positioned outsidethe acoustic substrate.
 27. The synthetic jet actuator of claim 25,wherein: the acoustic nozzle is a monolithically integral component ofthe cavity layer, and the flow path provided within the acoustic nozzlecomprises a flow expansion chamber.
 28. A method of making an actuatoraccording to claim 1, the method comprising: positioning the firstoscillatory membrane adjacent to a planar surface of the planar surfaceof the first cavity layer having the cavity opening and joining thefirst oscillatory membrane and the cavity layer by an adhesive material.29. The method according to claim 28, further comprising: forming theoscillatory membrane by joining the piezoelectric material to asubstrate.
 30. The method according to claim 28, further comprising:pre-stressing the oscillatory membrane during assembly through heatforming.
 31. The method according to claim 30, wherein: heat forming ofthe oscillatory membrane is performed via voltage compression.
 32. Themethod according to claim 28, further comprising: pre-stressing theoscillatory membrane during assembly by electrically actuating thepiezoelectric material.
 33. The method according to claim 28, furthercomprising: positioning an independent mass structure within theoscillatory membrane, adjacent the piezoelectric material.
 34. Themethod according to claim 33, wherein: the independent mass structure isjoined to the piezoelectric material with a high strength, low shearmodulus adhesive.
 35. The method according to claim 28, furthercomprising generating a vacuum pressure to apply a uniform atmosphericpressure to press the cavity layer and oscillatory membrane together.36. The method according to claim 35, further comprising: heat curingthe cavity layer and oscillatory membrane while apply the uniformatmospheric pressure.